High-accuracy polarimetry over fields of view spanning more than about 10 degrees in one, and in particular, in two dimensions is difficult to implement and requires highly customized optics and calibration schemes for every particular application. Some existing systems use wide-field optics that introduce large amounts of artificial polarization, and require anti-reflective coatings and/or substantial calibration.
Wide-field optical systems using lenses and/or mirrors (e.g. fish-eye lenses, Schwarzschild-type optical systems etc.) are typically complicated systems which suffer from optical distortions and field-dependent aberrations. In addition, these systems involve interfaces between air/vacuum and optical surfaces through which light rays pass at angles strongly deviating from normal incidence. Fresnel refraction and reflection at these surfaces introduces unwanted polarization that depends on the field position and can drown the small scene polarization to be measured. Special optical designs and coatings can reduce these effects, but not to a level that would be acceptable without significant calibration efforts.
Furthermore, polarization optics that could be placed before a wide-field optical system (i.e. to analyse the polarization of the light before it reaches the wide-field optics) are sensitive to the field angle because of their reliance on anisotropic materials. The issue of large incidence angles also affects such optics. As a result, wide-field polarization optics cannot meet stringent polarimetry performance requirements, and even extensive calibrations may not overcome this limitation.
The apparatus and method disclosed herein may address one or more of these issues.
The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the present disclosure may or may not address one or more of the background issues.